The present invention relates to devices for guiding drills, and particularly for guiding drills for boring centrally located holes in broken bolts so as to facilitate use of a bolt extractor to remove such broken bolts from threaded bores.
Because of corrosion, metal fatigue, and other related and cooperating factors, bolts often break leaving parts of them engaged in threaded bores. Often, the portion of such a broken bolt left exposed to be grasped is too little to be useful to remove the remaining part of the bolt from the threaded bore. It is often impractical in such situations to disassemble the structure or device in which the bolt is located to the extent necessary to permit precision machinery to be used conveniently to remove the broken bolt. Ordinarily, therefore, it is desirable at least to attempt to remove a broken bolt by the use of a bolt extractor, a wellknown tool having a tapered, fluted body twisted to provide steeply pitched multiple shallow threads of the opposite hand from that of the threads of the broken bolt. Such a tool is of strong, hardened metal, and its taper causes the steeply threaded portion to fit with increasing security within a hole bored centrally within the remaining portion of a broken bolt, so that the bolt extractor may be turned with an appropriate wrench to remove the remaining portion of the broken bolt from the threads of the bore in which it is engaged. The hole bored in a bolt to be removed in this manner must be located centrally in order to avoid damaging the threads of the hole in which the bolt is engaged, either while boring the hole or by use of the extractor, as damaging those threads would only add to the problems presented by the broken bolt itself.
Because the surface of the remaining portion of a twisted-off bolt is typically uneven, it is usually very difficult to start a hand-held drill into the remaining portion of a broken bolt to produce a centrally located hole extending coaxially with the remainder of the broken bolt.
While there are devices which might be useful to some extent for guiding a drill to produce such a hole in the remaining portion of a broken bolt, the previously available devices are undesirably complex and expensive, or else not well adapted for use in varied circumstances. For example, Moss U.S. Pat. No. 3,148,562 discloses a tool in which a pair of legs are hinged together and may be clamped to establish a required distance between opposite end portions of the device. A bolt may be placed through a bore in one end of the device into a threaded hole, while a drill guide is received in the other end of the device to establish the location and direction for a hole parallel to the bolt and at a desired distance. The Moss device, however, is apparently not useful except on workpieces having a flat surface between and perpendicular to parallel threaded holes. Cotter U.S. Pat. No. 3,083,593 discloses a drill guide apparatus for ensuring that holes are drilled perpendicularly into a workpiece at locations determined by a jig. Anderson U.S. Pat. No. 3,146,675 discloses a hole cutter in which an adjustably clamped center locator is movable along an arm of a drill guide holder. Comorau U.S. Pat. No. 3,026,748 discloses a drill guiding device including a complex jig arrangement. Whitmore U.S. Pat. No. 2,435,256 discloses an adjustable hole locating device which does not appear to be useful in drilling holes in broken-off studs in most locations because of its size. Berlin et al. U.S. Pat. No. 2,936,657 discloses a drill guide which depends on sharp edges engaging the surface surrounding the location where a hole is to be bored.
What is needed, then, is a simple device for guiding a drill to bore central holes coaxially into an end of a broken bolt, so that the bolt can be extracted without damaging the material surrounding it. Such a device should be of simple construction, inexpensive, and adaptable for use in a range of sizes.